It is known that when filtering liquids, filter aids (so-called "precoats") are often used in addition to the usual filter membranes and devices. Filter aids help to trap fine particles of foreign materials present in the liquids to be filtered. The solid particles contained in the liquids will adhere to the filter aid, while leaving enough open spaces for the filtrate to pass through. In addition, the filter aids can also be used to produce a filter cake prior to the filtration process. This filter cake is sufficiently permeable to permit liquid but not solid particles to pass through it. Additional quantities of the filter aid can also be added to the liquid to be filtered, in addition to the use of the filter cake.
The most commonly used filter aids are Kieselgur, perlite, and cellulose fibers, and these filter aids are considered to be superior to other filter aids (Ullman's Encyklopadie der technischen Chemie, 4th ed., Vol. 2 (1972), pages 195 and 196, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., Vol. 10 (1980) page 296.
It was found that when using Kieselgur and perlite as filter aids with alkaline solutions, in particular strong alkaline solutions, such filter aids were attacked by the solutions. This effect is very pronounced when filtering hot alkaline solutions, especially in a temperature range of about 80.degree. to about 100.degree. C. It can be shown that when filtering sodium aluminate solution, Kieselgur as well as perlite react with the aluminate solution to form sodium aluminum silicates. This causes the mesh of the filter, e.g. a wire mesh filter, to clog, and the filter cake will harden after contact with the solution within one day of its use, resulting in time consuming cleaning operations. Even cellulose fibers cannot withstand very strong alkaline solutions.
Coal can be used as a filter aid to produce clear strongly alkaline solutions; however, the price of powdered coal is much higher than that of Kieselgur or perlite. In addition, after having used coal as a filter aid, extensive cleaning operations in the filtering plant are required following the filtering step.
Hence, there is an unfilled need for a filter aid material that compares favorably, pricewise, to Kieselgur and perlite; and which does not clog the filter cake or the filter itself when filtering alkaline solutions, particularly hot strongly alkaline solutions.
Porous bodies of .alpha.-aluminum oxide are known to the art. German Patent No. DE-OS 24 09 634 describes a porous body of .alpha.-aluminum oxide with a crystalline structure; this body has a number of channels traversing its entire thickness. The diameter of these channels is smaller than 0.01 .mu.m in at least one section thereof. Such a body can be made of aluminates of the .beta.-aluminum oxide type. These aluminates are heated to a temperature just below the melting point of the aluminate, and above the sublimation temperature of the metal oxides comprising the aluminate. Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd ed., Vol. 2 (1978), page 237, mentions the application of sintered aluminum oxide as a filter aid for molten metal, and Ullmann's Encyklopadie der technischen Chemie, 4th ed., Vol. 7 (1974), page 327, describes the application of active aluminum oxides as filter aids.
None of these references suggest or describe the use of calcined .alpha.-aluminum oxide as a filter aid.